Add a gg_forrest function


Example implementations
https://sakaluk.wordpress.com/2016/02/16/7-make-it-pretty-plots-for-meta-analysis/ 
https://ecologyforacrowdedplanet.wordpress.com/2013/05/10/using-metafor-and-ggplot-togetherpart-1/

Ideally will allow easy clustering by design or outcome. 